Discover how to measure battery state-of-charge and what future developments may bring.
The life of a battery cannot be defined by the number of cycles or age alone but by how the battery is used. As the capacity fades, the discharge time gets shorter. The smart battery captures the changes but these vital health statistics often remain hidden from the user. This turns a battery into a “black box,” concealing the performance records and disguising when the battery should be replaced.
One of the main tasks of the smart battery is to establish communication between the battery and the user. A fuel gauge indicating state-of-charge fulfills part of this. When pressing the TEST button on a fully charged SMBus battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery all lights remain dark or a red light appears. Figure 1 shows a fuel gauge of a battery that is 75 percent charged with three lights glowing.
Figure 1: State-of-charge readout of a “smart” battery
Signal lights indicate the battery SoC when pressing the TEST button.
Courtesy of Cadex
While the SoC information displayed on a battery or a display screen is helpful to the user, the readout does not guarantee runtime. The fuel gauge resets to 100 percent on a full recharge regardless of how much capacity the battery can store. A serious breach can occur if an aged battery shows 100 percent SoC while the battery’s ability to hold charge has dropped to 50 percent or less. We ask, “100 percent of what?” If, for example, 100 percent of a good battery results in a four-hour runtime, a battery holding half the capacity would run for only two hours. Many users are not aware that the fuel gauge only shows SoC; capacity, the leading health indicator, remains unknown.
Other than applying a controlled discharge, there is no reliable method to measure the capacity of the “chemical battery,” but there is a way to estimate the “digital battery.” Chemical battery refers to the capacity derived by discharging a fully charged battery; digital battery is the estimated capacity through coulomb counting when charging and discharging the battery while in use.
As part of permanent data, the SMBus battery is programmed with the specified capacity, which is 100 percent by default. With each full charge, the battery resets to the full-charge flag; and during discharge the coulomb counter measures the consumed energy. A perfect battery delivers 100 percent, but as the battery ages and the capacity drops, the energy decreases. The difference between the factory-set 100 percent and the delivered coulombs is the full charge capacity (FCC). FCC reflects the digital equivalent of a full discharge, or digital capacity.
Coulomb counting can also estimate SoH during charging, and this works best with an empty battery. A battery with a 100 percent capacity will receive the full coulomb-count; one with only 50 percent capacity will accept only half before the battery reaches full-charge.
Not knowing the exact SoC when the coulomb count begins will result in low accuracies. SoC can be estimated by measuring the battery’s open circuit voltage (OCV), but this only gives a rough approximation, as agitation after charge or discharge and temperature affect the OCV. Periodic calibrations improve accuracy of the digital battery.
The SoC and capacity information can be shown on a linear display using colored LEDs. The green lights indicate the usable capacity; the empty part of the battery is marked with un-lit LEDs; and the unusable part is shown with red LEDs. Figure 2 illustrates a tri-state fuel gauge. The results can be a shown on a digital display.
Figure 2: Tri-state fuel gauge.The tri-state fuel gauge reads the “learned” battery information on the SMBus and displays it on a multi colored LED bar. The illustration shows a partially discharged battery of 50% SoC with 20% empty and 30% unusable.
Courtesy of Cadex
The tri-state fuel gauge provides state-of-function (SoF), the ultimate in battery diagnostics. Device manufacturers are hesitant to offer a feature to consumers that could lead to higher warranty claims. A replacement becomes mandatory if the battery capacity drops below 80 percent. Keeping the evidence hidden is seen as the least disruptive method. SoF can always be accessed by a service code.
Vehicles with electric propulsion do not show the charge but the remaining driving range, thus hiding the capacity. To accommodate capacity fade that would shorten the driving range, the oversized EV battery does not get a full charge and discharge when new. Only as the battery ages and the capacity fades will the charging range gradually increase. Shorter driving ranges will only become apparent once this grace buffer has been consumed. (See BU-1003: Electric Vehicle.)
Last updated 2015-11-26
Comments are intended for "commenting," an open discussion amongst site visitors. Battery University monitors the comments and understands the importance of expressing perspectives and opinions in a shared forum. However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination.
If you have a question, require further information, have a suggestion or would like to report an error, use the "contact us" form or email us at: BatteryU@cadex.com. While we make all efforts to answer your questions accurately, we cannot guarantee results. Neither can we take responsibility for any damages or injuries that may result as a consequence of the information provided. Please accept our advice as a free public support rather than an engineering or professional service.